Recently, according to the so-called green house effect, which is the result of carbon dioxide discharge, a global warming phenomenon is rapidly progressing, so that a serious natural disaster occurs, thereby the existence of human beings is threatened. Accordingly, human beings have become interested in hydrogen energy, which does not harm the environment, as a source of clean energy, and research and development into the clean energy has been focused on methods of economically producing hydrogen.
A process for thermochemically producing hydrogen referred to as an Iodine-Sulfur cycle is considered the most efficient of the methods of producing hydrogen. In the process, hydrogen is produced by thermally decomposing sulfuric acid using a high-temperature gas cooling furnace. The above process has been considered to be an influential method in that heat is stably supplied at a temperature of 950° C. or more and dangerousness is low. However, the selection of the material used in an apparatus for performing the process is becoming the most important issue. The reason is that a metal material must be used for high-temperature elasticity in the apparatus for the hydrogen producing process, but SO2 and SO3, generated at the time of thermally decomposing sulfuric acid, have extremely high corrosiveness, and thus it is difficult to establish an economical system using any metallic material that has been developed to date, and ceramic materials have excellent corrosion resistance but can be broken by thermal stress at high temperature, so that it is difficult to use such ceramic materials in the apparatus for the hydrogen producing process. Accordingly, a method of coating the ceramic to a metallic base material having an excellent thermal property at high temperature has been proposed.
However, generally, since ceramics and metals are different from each other in the thermal expansion, the thermal conductivity, and the like, they have poor adhesiveness with each other and thus are easily separated. One of the reasons is that, when a metal is exposed to an atmosphere with a high temperature, an oxide film is easily formed on the surface thereof, so that, when the metal is coated with different materials, the oxide film decreases adhesiveness therebetween.
While the present inventors researched methods of increasing adhesiveness between a metal base material and a ceramic thin film and maintaining high adhesiveness even at high temperature, they found that, when the ceramic is mixed with the metal materials at an interface therebetween using a so-called ion beam mixing method, which is a method of coating a metal base material with a ceramic thin film and then mixing the two different materials by radiating an ion beam, the adhesiveness is increased, and when a ceramic thin film is further applied to the mixed layer, the adhesiveness is maintained even at high temperatures, and the corrosion resistance at a high temperature is improved. As the result of the findings, the present inventors completed the present invention.